Add LEN() general purpose length function to query docs.

Added function:

LEN( <value> )

The result is a number depending on the type of value passed in, for:

• string - string length (equivalent to LENGTH())
• object - field count (equivalent to OBJECT_LENGTH())
• array - number of elements (equivalent to ARRAY_LENGTH())
• binary - size of binary object
• boolean - 1
• number - the number of characters in the number's text representation
• missing - missing
• null - null
• Anything not in the list - null